Centrifugal furnace



A ril 9, 1968 E. R. CAPITA 3,377,417

CENTRIFUGAL FURNACE Filed April 50, 1965 2 Sheets-Sheet 1 INVENTCR. 57/; 9 CAP/779 $7M)? b n-M April 9, 1968 E. R. CAPITA 3,377,417

CENTRIFUGAL FURNACE Filed April 30, 1965 I 2 Sheets$heet a INVENTOR [Iv/L C/lP/r l JZMW United States Patent 3,377,417 CENTRIFUGAL FURNACE Emil R. Capita, 7020 Hudson Blvd., North Bergen, NJ. 07047 Filed Apr. 30, 1965, Ser. No. 452,268

3 Claims. (Cl. 13-1) ABSTRACT OF THE DISCLOSURE A method and apparatus for melting or sintering powdered refractory materials to transform them to an even density solid state using a container made from a material having a lower melting point than the refractory material. The melting container is rotated to produce a centrifugal force for holding the powdered material in a uniform density layer against the container walls and only the innermost portion of the refractory material is melted by means of an inductive heating coil. A protective boundary layer of unmelted refractory material remains between the melted refractory material and the container walls thereby avoiding contamination of the melted portion of the refractory material by the container material during extreme heating.

The present invention relates to an improved sintering furnace and more particularly to such a furnace for sintering or melting frit of highly refractory materials.

Numerous highly refractory materials such as molybdenum, tungsten, tantalum, titanium and zirconium are first produced in powdered form. Before these materials can be converted into useful form it is necessary to melt or sinter the powder to transform it to a solid state. Considerable difliculty has heretofore been encountered in the melting since extremely high melting or sintering temperatures are required above those which melt conventional containers. Practical containers permitting a melting of the powder in pure form and without contamination have not been available. It has been found impossible to melt these materials in any sort of previously known container or support without serious contamination of the materials by the transfer of contaminating matter due to the high temperatures required at the sur faces of the supports or containers used.

The present invention provides a novel method and apparatus for melting or sintering the powders with no contamination and wherein conventional materials may be used to provide the necessary support even at the extremely high temperatures necessary to effect the sintering or melting.

This new method in general comprises using centrifugal force to hold the powdered material at the inner surface of a rotating element before and during the melting of the powder and insures the elimination of any contamination being transferred from the rotating support to the powdered material by causing only the inner-most portion of the material to be melted so that a protective boundary layer of the powdered material remains between the melted portion and the support. This boundary layer not only prevents the transfer of contaminating material from the support to the powder but it also acts as a thermal insulating layer so that the rotating support itself may be at a temperature well below that required for melting the inner layers of the powdered material. Thus, for example, the rotating support itself may be made of stainless steel and the inner melted layer of the powdered material'may have its temperature raised considerably above the melting point of the support due to the insulation provided by the boundary layer of unmelted powdered material.

Accordingly, an object of the present invention is to provide a new method of melting or sintering highly refractory materials.

Another object of the present invention is to provide a method and means of melting or sintering materials and particularly powders with extremely high melting points without contamination.

Another object of the present invention is to provide a novel means for supporting and melting high temperature materials having extremely high melting points.

Another object of the present invention is to provide a novel method of melting powdered materials using induction heating.

Other and further objects of the invention will be obvious upon an understanding of the illustrative embodiment about to be described or will be indicated in the appended claims, and various advantages not referred to herein will occur to one skilled in the art upon employment of the invention in practice.

A preferred embodiment of the invention has been chosen for purposes of illustration and description and is shown in the accompanying drawings, forming a part of the specification, wherein:

FIG. 1 is a Vertical sectional view of the furnace;

FIG. 2 is a sectional view taken along line 22 of FIG. 1;

FIG. 3 is an enlarged fragmentary sectional view of another embodiment;

FIG. 4 is a vertical sectional view of another embodiment of the furnace;

FIG. 5 is a horizontal sectional view taken along line 55 of FIG. 4; and

FIG. 6 is a side elevational view of an additional embodiment.

The method and furnace of the present invention are useful with a variety of highly refracted materials such as the metals mentioned above as well for sintering other powdered materials such as powdered carbon including powdered carbide.

FIG. 1 shows a centrifugal furnace 1 in accordance with the present invention. This furnace 1 comprises a hollow cylindrical melting chamber 2 with its walls formed of a suitable metal such as stainless steel or of graphite or other high temperature oxide refractories. As will be apparent from the following description, the melt ing chamber 2 including :the bottom 3 and the mounting shaft 4 may be made of a material having a melting point considerably below that of the powdered material to be melted in the furnace.

The melting chamber 2 is rotated about the axis of the shaft 4 at relatively high speeds thereby creating centrifugal forces on powdered material 5 supplied to the chamber 2 and which cause the powder to arrange itself in a layer of relatively uniform thickness on the rotating inner walls of the chamber 2. Depending upon the density and particle size of the material to be melted, the melting chamber is rotated at speeds from several hundred rpm.

to several thousand rpm. The rotational speed required is readily ascertained from an observation of the disposition of the powdered material within the chamber 2 and the subsequent melting and the rotational speed need not be raised above a speed at which the powder and the melted portion are observed to arrange themselves in relatively uniform layers on the inner walls of melting chamber 2 and as illustrated in FIG. 1.

In order to prevent contamination by the furnace atmosphere, the melting chamber preferably is contained within an outer vacuum chamber 6 which may either be evacuated or filled with an inert atmosphere under pressure during the operation. This entire chamber including the support bearings 7 for the shaft 4 is movably mounted on suitable pivots or otherwise to permit the melting chamber to be tipped at the end of the melting operation the rotating 3 so that the melted material may be poured into suitable crucibles or molds within or without the chamber after the melting operation. Where tubular form is desired, the melted material is cooled in position on the chamber 2 walls as rotation is continued.

A preferred embodiment of a drive is illustrated for such an air-tight chamber which includes a magnetic coupling with connecting magnets 8 and 9 transmitting the rotational force through the chamber wall 6 and with a suitable variable speed drive motor 10 operatively coupled to the lower drive magnet 9.

The powdered material to be melted preferably is admitted to the melting chamber 2 by an adjustable feed nozzle 11 arranged so that a coating of relatively uniform thickness of the powder 5 may be arranged along the inner wall of chamber 2 starting from the bottom 3 and moving toward the open end of the chamber 2. This may be conveniently done by bringing the melting chamber 2 up to its desired rotational speed and by thereafter admitting the powder as a powder feed nozzle 11 is raised along the side wall of the chamber 2 with the amount of powder being controlled to provide a cylindrical coating of the desired thickness. The entire layer of powder may be admitted before the melting or the melting may be commenced after a portion of the powders has been admitted adjacent to the bottom 3 of the chamber 2.

After the powder 5 has been fed into the melting chamber 2 and is positioned by centrifugal force in a layer against the rotating chamber walls, the innermost portion of the powder layer is heated and melted. The heat is applied in controlled amounts to limit the melting to a portion only of the powder thickness. In this way, the melted material only contacts a layer of the same material in powdered form and thus cannot be contaminated by the supporting chamber. In addition, the remaining unmelted portion acts as an insulator to reduce heat transmission to the walls of the melting chamber 2 so that these walls remain at a temperature considerably below that of the melted material and thus may be of a material having a much lower melting point than that of the material being treated.

While a variety of means of applying the heat may be used, the preferred melting element comprises an induction heating coil 12. This coil includes several circular turns positioned adjacent to the inner edge of the powdered coating and movably mounted on suitable current leads 13 so that the coil 12 may be moved axially of the rotating chamber 2. Induction heating is particularly suitable since a wide variety of materials of the highly refractory type conduct current and as the frequency and current intensity of the induction coils may be adjusted to provide the amount of heat necessary to melt only a portion of the powdered depth as described above. Additionally, induction heat is particularly useful in this regard due to the well-known skin effect of induced currents wherein the induced currents tend to concentrate themselves near the surface of the powdered layer so that the heating effect is concentrated in this area. This concentration of the heating effect provides the result desired as the radially inner-most portion of the powder melts first and as control is easily retained over the depth of the powder melted and over the thickness of the barrier layer of the unmelted powder remaining between the melted portions and the walls of the melting chambet 2.

In the embodiment illustrated in FIG. 1, the depth of the unmelted layer may be visually observed when the powder is fed slightly above the position of the induction heating coil 12. The frequency of the induction heating system current as supplied by an alternating current source 14 has been found to be variable between 500 cycles per second and 5 megacycles per second. Higher frequencies tend to increase the skin effect or to concentrate the heating currents near the surface of the powder. The higher frequencies thus are preferred as they facili- 4 tate the control of the depth of the melted powder. The exact frequency used is not critical and the amount of powder required and the frequency are easily determined by a few trial runs and by observation of the results. The proper melting conditions thereafter may be duplicated for successive melting operations.

As indicated above, this method is particularly useful for highly refractory metals since it provides a contamination free method and also permits high strength materials having relatively low melting points to be used for the melting chamber. The method may also be used advantageously for other materials such as carbon or carbide to form tubes or other forms. In this case, the melting chamber itself may be made of carbon so that there is no contamination problem and the heating may also be applied from the outside of the chamber by suitable flames or induction heating coils encircling the chamber as illustrated at 15 in FIG. 3. In such a melting operation, both an inner and outer induction heating coil may be used permitting a portion of the necessary heat to be supplied from the outside and the final melting heating to be supplied by the inner coil 12. In this way the carbon chamber itself may still be kept at a lower temperature than that of the melted carbon particles.

This centrifugal sintering procedure is particularly useful in manufacturing carbide tubing as it produces such tubing with a uniform density. The uniform density results from the uniformity of the centrifugal force throughout the rotating material during the sintering process. Presently known methods of producing carbide tubing do not produce this result. The presently used methods heat carbide granules and press them with a ram. The inherent friction in the granules results in a greater density in the tubing adjacent to the ram than is obtained in portions more remote from the pressing ram.

The new method thus provides superior carbide tubes having a uniform density throughout. One example of a use for such a tube is an outer layer for the rollers in a rolling mill for producing sheet steel. In this use, a carbide tube manufactured in accordance with the new method is precision ground on its inner and outer surfaces and brazed in position over a solid steel mandrel. The resulting roller has a carbide coating of uniform density and thus has uniform physical properties throughout its entire length and provides a superior roller for high temperature applications.

At the termination of the melting operation, as described above, the heat may be removed and the melted material permitted to solidify as a hollow tube or the molten material may be poured out by slowing down the rotational speed of the chamber and tilting it and pouring the melted material into suitable molds or alternatively a solid ingot of the material may be formed by permitting it to flow into the bottom of the melting chamber from which it is subsequently removed.

While the material has been described as being in powdered form it is clear that it may be either a fine grained powder or be in the form of frit or other form having a relatively large particle or pellet size as long as it may be fed into the chamber 2 and formed into a coating of the general form described.

FIGS. 4 through 6 illustrate additional embodiments of the furnace particularly adapted for producing products such as clear quartz tubes. In the embodiment of FIGS. 4 and 5 a hollow carbon melting chamber 20 is' mounted for high-speed rotation on a shaft 21. A poW- dered material 22 is supplied to the interior of the chamber 20 and the rotation of the chamber causes a layer of the material 22 to cling to the inner periphery 23 of the chamber 20. A layer of clear quartz sand, for example, may be admitted to the chamber 20 and upon the heating of the rotating carbon chamber 20 with a heat source such as the induction heating coil 24, the quartz sand will melt or fuse into a clear homogenous mass in accordance with the shape and size of the mold.

Upon removal of the heat source from the chamber 20, the melted mass will harden into a clear quartz tube. In some cases the fused material may be poured from the mold and hardened in other mold shapes.

The preferred embodiment of the chamber 20 illustrated has a corrugated shape in cross-section as seen in FIG. 5. The corrugations 25 provide flexibility for the chamber 20 walls to accommodate them to the particular coefiicient of expansion for the material being fused within the chamber 20 such as, for example, where thin quartz tubes are being formed in a chamber 20 made of carbon.

FIG. 6 illustrates another embodiment with a chamber 26 having a short induction heating coil 27 for use where the power supply is limited or Where a relatively long article is being formed by the operation. In this case the coil 27 heats the chamber 26 and the powdered material therein in successive increments as the coil 27 is moved axially of the rotating chamber 26.

It will be seen that an improved method and means for melting or sintering powdered material have been provided wherein contamination has been eliminated. The method also permits the use of high strength support materials for the melting chamber which support materials may have a melting point lower than that of the refractory materials being handled. These improvements provide a process for sintering highly refractory materials not heretofore capable of being handled in a convenient, eflicient, and relatively inexpensive manner. The method of the invention is particularly useful in View of its relative simplicity and in view of its capability for precise and continuing control which make it capable of being performed both in relatively small laboratory sized equipment and also in larger commercial installations operated by automatic control. The method and means of the invention are capable of melting or sintering a variety of highly refractory materials with a degree of purity not heretofore consistently obtainable.

As various changes may be made in the form, construction and arrangement of the parts herein without departing from the spirit and scope of the invention and without sacrificing any of its advantages, it is to be understood that all matter herein is to be interpreted as illustrative and not in a limiting sense.

Having thus described my invention, I claim:

1. A centrifugal furnace comprising the combination of a hollow generally cylindical melting chamber, means of mounting said chamber for rotation about the cylindrical axis of said chamber, means for feeding a poW- dered electrically conductive material to the interior Wall of said chamber, induction heating means movably mounted in said chamber for motion along the cylindrical axis for heating only the innermost portion of the material, and drive means for continuously rotating said chamber for causing centrifugal force to hold said material against said interior wall during the heating.

2. The furnace as claimed in claim 1 in which said means for feeding the powdered material comprises a movably mounted nozzle positioned for applying the material in a coating of predetermined thickness extending in an axial direction along the interior wall of the chamher.

3. The furnace as claimed in claim 1 which further comprises a sealed outer chamber surrounding said melting chamber for containing a selected atmosphere around the melting chamber.

References Cited UNITED STATES PATENTS 2,341,739 2/1944 Olt et al 2l9-l0.73 X 2,551,360 5/1951 Bierwirth 2l910.73 2,870,309 1/1959 Capita 219l0.73 X 3,248,215 4/1966 Bonis et al. 219-1073 X RICHARD M. WOOD, Primary Examiner. L. H. BENDER, Assistant Examiner. 

